We have shown earlier that a group or repeated sequences which are homologous to the ribosomal DNA insertion occur in the chromocentric heterochromatin of Drosophila melanogaster. Several cloned DNA fragments have been isolated which contain regions homologous to the insertion interspersed with other DNA sequences called flanking sequences. We have shown that some of the insertion-like sequences occur in tandem repeats within the chromocenter. An analysis of several flanking sequences showed that some are transcribed into heterogeneous nuclear RNA. The chromosomal location of flanking sequences is being studied in different strains and species of Drosophila.